1. Field of the Invention
The present invention relates to a developing apparatus incorporated in an electrophotographic copier, printer, facsimile transceiver or similar image forming equipment. More particularly, the present invention is concerned with a developing apparatus of the type depositing a developer on a developer carrier and transporting it to a developing position where the developer carrier faces an image carrier to thereby develop a latent image electrostatically formed on the image carrier.
2. Discussion of the Background
A developing apparatus of the type described is disclosed in, for example, Japanese Patent Publication No. 32375/1983 and includes an image carrier and a developer carrier located face-to-face at a developing position. An alternating electric field is generated in the developing position to repetitively transfer a developer from the developer carrier to the image carrier and from the image carrier to the developer carrier, thereby developing an electrostatic latent image formed on the image carrier. In this type of apparatus, the developer carrier is implemented as a cylindrical nonmagnetic sleeve accommodating a permanent magnet in the form of a roll. A magnetic toner contacts such a developer carrier due to the force of the magnet and gravity. The toner is charged to a predetermined polarity by friction thereof with the surface of the nonmagnetic sleeve. As the toner retained on the sleeve by the force of the magnet reaches a position where a magnetic blade faces the sleeve at a predetermined spacing, it is regulated by the blade to form a layer which is about 70 .mu.m.
The problem with the conventional apparatus described above is that the magnetic toner cannot be sufficiently charged since the toner contacting the nonmagnetic sleeve due to the force of the magnet and gravity is simply charged by friction in contact with the sleeve being rotated. For example, the amount of charge on the toner forming the second layer and successive layers as counted from the surface of the sleeve is extremely small. The toner with a comparatively small amount of charge easily flies toward the image carrier due to the alternating electric field and, therefore, enhances the tonality of an image. However, such a toner is apt to contaminate the background of the image carrier to thereby thicken lines of an image and lower the resolution.